fix(sheets-ui): support Ctrl+Click for context menu on macOS

[StTronn]

Fixes #5976

On macOS, Ctrl+Click is functionally equivalent to right-click but was not
recognized by Univer. This commit adds support by checking for Ctrl+Click
(button 0 with ctrlKey) in addition to right-click (button 2).

Changes:

• Update all three pointer down handlers (main area, row/col headers)

Description

On macOS, Ctrl+Click is functionally equivalent to right-click but was not recognized by Univer's context menu system. This PR adds support for Ctrl+Click to open context menus, matching the behavior of native macOS applications
like Chrome and Safari.

How to test

1. Open any Univer spreadsheet
2. Select a cell
3. Hold Ctrl and click on the cell (Ctrl+Click)
4. ✅ Context menu should appear
5. Also verify right-click still works normally
6. Verify normal clicks without Ctrl are not affected

On Windows/Linux:

1. Verify right-click context menu still works as before
2. Verify Ctrl+Click does NOT trigger context menu (expected behavior)

Pull Request Checklist

• Related tickets or issues have been linked in the PR description (or missing issue).
• Naming convention is followed (do please check it especially when you created new plugins, commands and resources).
• Unit tests have been added for the changes (if applicable).
• Breaking changes have been documented (or no breaking changes introduced in this PR).

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